A typical electroplating system consists of a cathode, an anode, and an electrolytic solution. The cathode is the work piece upon which metal is to be plated, and the anode functions as the counter-electrode in the electrochemical cell. The electrolytic solution contains dissolved metal ions along with other constituents which influence deposit quality. The cathode and anode are immersed in the electrolytic solution and connected by a power supply. A voltage difference is applied between the cathode and anode, and current flows freely from the anode to the cathode.
At the cathode surface, metal is deposited as metal ions are reduced to their base form via an electrochemical reaction: M.sup.+v +v e.sup.- .fwdarw.M.sup.0. To conserve charge, an electrochemical reaction also occurs at the anode surface and can be one of two types. If the anode is soluble at the potential being applied, it dissolves and releases metal ions into solution: M.sup.0 .fwdarw.M.sup.+v +v e.sup.-. If the anode is insoluble at the potential being applied, a gas evolution reaction, such as 2 O.sup.-2 .fwdarw.O.sub.2 +4 e.sup.-, occurs at the anode. A variety of other side reactions are also possible at both the cathode and the anode.
In electrolytic copper plating, the actual properties of the deposited metal are a strong function of local agitation, current density, and the exact concentration of all bath components, including organic additives. It is well known that bright, smooth copper deposits cannot be obtained without the presence of organic additives. Such additives must be controlled during production in order to obtain consistent metallurgical properties, including grain structure, brightness, smoothness, leveling, and purity. The degree to which various additives must be controlled is a strong function of the application at hand. Perhaps the most demanding applications lie within the microelectronics industry, where very small metal features need to be synthesized, without irregularities or surface anomalies.
Several of the common additives, including a copper brightener and grain refiner sold under the trademark CuBath M-D by Enthone-OMI Corporation, are easily oxidized at the bare anode surface. This electrochemical degradation can cause a continuous depletion of the organic additives which can lead to poor metal quality if not properly controlled. On the other hand, increased stability of the organic additives leads to longer lifetimes of the electroplating baths which is economically very important. For example, frequent replacement of the bath interrupts the copper plating operation which reduces product yield and requires replacement of the chemicals in the new bath as well as disposal of the chemicals in the old bath. Accordingly, there is a need for a device, process, or additive which would stabilize organic additives within an electrolytic solution to preserve deposit quality and extend bath life.
Efforts along these lines have been made. For example, some attempts have been made to control additive degradation by separating the anode from the bulk solution by using a membrane. Membranes that restrict the passage of additives usually also restrict passage of copper ions, which can cause over-potential problems at the anode surface. This problem can only be combated with a complex exchange scheme within the anode chamber. Other efforts have focused upon implementing steady-state bath exchange schemes, in which old solution is discarded to remove harmful breakdown products, and new solution is added to replenish additives. Bath exchange schemes are viable, but are clearly more cumbersome and costly than preventing the problem at the outset.
The breakdown of organic additives in the presence of copper can be significantly retarded by forming a protective film on the anode surface. However, an additional problem is encountered when the particular cathode to be plated requires that a relatively low cathode current density be used. In these cases, forming such a protective film over the anode surface has been accomplished only with difficulty. More specifically, the areas of the anode remote from the cathode can only be completely filmed by increasing the current density, which might not be possible due to the product requirements of the cathode. When subsequently plating copper in a system having an anode which has only been partially covered with a protective film, the organic additives tend to be consumed at the unprotected anode surface.